This invention generally relates to a process for accomplishing the development of images in electrostatographic imaging systems, and more specifically, the present invention is directed to an improved process for accomplishing the development of electrostatic latent images, by providing a development zone situated between a deflected flexible imaging member and a transporting member, wherein the imaging member is deflected by conductive developer particles, comprised of toner particles and conductive contacting carrier particles contained in the development zone, which deflection is primary responsible, together with the movement of said members, for the agitation and movement of developer particles. A process utilizing conductive developer materials allows the continual development of high quality images, including the efficient and effective development of solid areas.
The development of images by electrostatographic means is well-known, and in these systems, toner particles are applied to an electrostatic latent image to be developed by using, for example, various development methods including cascade development as described in U.S. Pat. No. 3,618,552, magnetic brush development as described in U.S. Pat. No. 2,874,063, and powder cloud development as described in U.S. Pat. No. 2,217,776. Powder cloud development and cascade development methods have been found to be especially well-suited for the development of line images common to business documents, however, images containing solid areas are not generally faithfully reproduced by these methods. The development of magnetic brush systems, however, provided an improved method for producing both line images and solid areas.
In magnetic brush development systems it is generally desirable to attempt to regulate the thickness of the developer composition which is transported on a roller, by causing the roller to move past a metering blade. Metering blade adjustment is important since the development zone the flow of developer material is determined by a narrow restrictive opening situated between the transporting roller and the imaging surface. Accordingly, in order to provide sufficient toner particles for the imaging surface, it is generally necessary to compress the developer bristles, thereby allowing toner particles adhering to the carrier particles near the ends of the bristle to be available for development. Any variation or non-uniformity in the amount of developer material metered onto the transport roller, or into the space in between the roller and imaging member, can result in undesired developer flow and non-uniform image development. The non-uniform image development can be minimized by carefully controlling the run-out on the developer roller and on the imaging member; and by providing a means for side to side adjustment in the relative positions of the metering blade, the development roller, and imaging member. When the imaging member is a flexible photoconductive belt, an improvement in the uniformity of development can be obtained by maintaining that portion of the belt in a slackened or non-tensioned condition so that the belt is capable of moving freely toward and away from the developer roller in response to the varying contours thereof as disclosed, for example, in U.S. Pat. No. 4,013,041.
The developer materials presently utilized in magnetic brush development systems differ widely in their electrical conductivity, thus, at one extreme in conductivity such materials can be insulating, in that a low electrical current is measured when a voltage is applied across the developer. Solid area development with insulating developer compositions is accomplished by metering a thin layer of developer onto a development roll, which is in close proximity to the image bearing member, the development roll functioning as an electrode and thus increasing the electrostatic force acting on the developer particles.
Insulating developer compositions can be rendered conductive by utilizing a magnetic carrier material which supports a high electric current flow in response to an applied potential. Generally, the conductivity of developer compositions depends upon a number of factors, including the conductivity properties of the magnetic carrier, the concentration of the toner particles, the magnetic field strength, the spacing between the imaging member and the development roll, and developer degradation due to toner smearing on the carrier particles. Also, when insulative toner particles are permanently bonded to a conductive carrier, the conductivity decreases to a critical value below which solid area development becomes inadequate, however, within certain limits the process and material parameters can be adjusted somewhat to recover the decrease in solid area developability. As indicated hereinafter, conductive magnetic carrier particles which render the developing composition conductive are employed in the process of the present invention.
It is known that when employing conductive developer materials in electrostatographic imaging systems that the development electrode members are maintained at a close effective distance from the image bearing member, wherein a high electrostatic force acts only on those toner particles which are adjacent to said members. Thus, since the electrostatic force for development in such systems are not strongly dependent upon the developer layer thickness, the uniformity of solid area development is improved despite a variation in the spacing between the image bearing member and the development roller member. While solid area deposition is not limited by a layer of net charged developer material near the imaging surface in magnetic brush systems utilizing conductive developer material, primarily since the charge is dissipated by conduction to a development roller, solid area deposition is limited by image field neutralization, provided there is sufficient toner available at the ends of the developer brush, which toner supply is limited to the ends of tips of the bristles, since toner cannot be extracted from the bulk of the developer, where high developer conductivity collapses the electric field within the developer at any location and confines it to a region located between the latent image and the developer composition. For either insulative or conductive developers, solid area deposition is limited by toner supply at low concentrations, and the toner supply is further limited to a layer of carrier material adjacent to the image bearing member, since the magnetic field stiffens the developer, and hinders developer mixing in the development zone.
In many of the above described systems, undesirable degradation or deterioration of the developer particles results, which is generally caused by a number of factors, including, for example, the frequency of collisions between adjacent carrier particles contained in the developer compositions, which collisions adversely effect the developer conductivity, and also the triboelectric charging relationships between the toner particles and the magnetic carrier particles. This degradation, which most likely occurs in the development roller, at the metering and development zone, and in the developer supply reservoir in which fresh toner is added and triboelectrically charged by carrier particles, adversely effects the developer conductivity, and the triboelectric charging relationships between the toner particles and the magnetic carrier particles. Thus, a decrease in the triboelectric charge on the toner particles causes an increase in solid area development, and an increase in the amount of toner particles that are deposited in the background areas. Accordingly, in order to maintain the original image quality in such situations, the triboelectric charge on the toner particles is increased by reducing the concentration of such particles in the developer composition mixture. Further, when the toner charge and toner concentration decreases, the developer material must be replaced in order to obtain images with acceptable solid areas and decreased background areas.
There is also known as disclosed in related U.S. Pat. Nos. 4,394,429 and 4,368,970, a system for causing the development of images in electrostatographic imaging devices, which system utilizes a deflected flexible imaging member and insulating developer particles.
While several improved types of processes and systems have been developed for the purpose of developing images, difficulties continue to be encountered in the design of a simple, inexpensive and reliable two-component conductive development system which provides a highly solid area development rate, low background deposition, and long-term stability. Accordingly, there continues to be a need for processes which will improve the quality of the images produced, particularly in electrostatographic imaging systems, such as xerographic imaging systems, which are simple and economical to operate, and which result in reproducible high quality images, including both line and solid area image development. Additionally, there continues to be a need for the provision of a process employing contacting conductive carrier particles in order that charges can be rapidly transferred to the flexible imaging member; and wherein background development is substantially eliminated, and the useable life of the developer composition is increased.